1. Field
The disclosed method of making, apparatus, and article of manufacture relates generally to high performance energy storage devices, and particularly to increasing electrode interface efficiency and lowering equivalent series resistance of such interfaces.
2. Related Art
Double layer capacitors, also referred to as electrochemical double layer capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. Additionally, they can typically deliver the stored energy at a higher power rating than rechargeable batteries.
Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to ionic) current from shorting the cell. Coupled to the back of each of the active electrodes is a current collecting element. One purpose of the current collecting element is to reduce ohmic losses in the double layer capacitor.
Specifically, improvements are needed in the techniques and methods for fabricating double layer capacitor electrodes so as to lower electrode resistance of the double layer capacitor and maximize the operating voltage. For example, the method used to connect the current collector element of the capacitor to the electrode is important because the interface between the electrode and the current collector element is a source of internal resistance of the double layer capacitor. Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. Equation 1 shows a mathematical expression for energy stored in a capacitor, wherein energy is measured in joules. Equation 2 below shows a mathematical equation for average power output of a capacitor in watts. It is apparent that improved techniques and methods are needed to lower the internal resistance of the electrodes used within a double layer capacitor and increase the operating voltage.E=CV2/2   Equation 1:Pav=CV2/2t   Equation 2:
There is a continuing need for improved double layer capacitor design. Such improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and energy density ratings within a relatively short period of time. Such improved double layer capacitors should also have a relatively low electrode interface equivalent series resistance (ESR) and yet be capable of yielding a relatively high operating voltage.
An ESR rating for a capacitor is a rating of quality. A theoretically perfect capacitor would have an ESR of zero. Ideal capacitors therefore have exactly 90 degree phase shift of voltage with respect to current which implies zero dissipation factor (“DF”). However, all real capacitors have some amount of ESR. Hence, a real-world challenge for capacitor designers is minimizing ESR. ESR is modeled like a resistor in series with a capacitor. Capacitor designs that appear optimally functional in theory, can fail when manufactured due to ESR. Increasingly, modern electronic designs rely on low ESR capacitors to function optimally in a real-world environment. Low ESR means low charge and discharge time constant which is very important in applications that require high power to energy ratios, such as hybrid electric vehicles, electric power assist steering, brake system support, and most industrial applications.
An interface point of termination from a capacitor electrode foil to an end cap, such as a terminal cap, is an issue in assembly and manufacturing. Modern laser welding techniques, for example, may only fuse 15%-40% of the available foil wraps inside a double layer capacitor, leading to part to part manufacturing variability in ESR, as well as loss of integrity with age.
Therefore, a need exists to improve consistency of manufacturing variability, and therefore improve manufacturing yield, as well as improve reliability of an energy storage device, such as for example a double layer capacitor, as it ages. The present teachings provide a method for making an electrode termination interface for solving the aforementioned problems and issues by providing a highly reliable, low cost, solution to improve electrode termination interface ESR, which is more efficient than prior art solutions.